1. Field of the Invention
The present invention relates generally to a molding mold for molding resin material and, more particularly, to an improvement of a gate portion of the molding mold in an open type hot-runner system, and further to a molding method using the molding mold, an injection molding apparatus to which the molding mold is added, and a product molded by the molding mold.
2. Description of the Prior Art
FIG. 8A is a plan view showing a half shell of a floppy disk molded by a molding mold adopting an open type hot-runner system in the prior art. The half shell 51 includes recessed portions 52, 53 formed in a fitting portion 51a for fitting a shutter plate. The recessed portions 52, 53 are formed so that a gate residual (a protruded portion) , if produced upon mold releasing, may not protrude from a surface 54 of the shutter plate fitting portion 51a. When actually molded, however, a gate residual portion (resin residual) 55 produced, as illustrated in FIG. 8B, extends from the recessed portion 52 and protrudes from the surface 54, resulting in causing high gate. A gate diameter of a gate portion, through which molten resin flows into a cavity, was 0.7 mm.
If a size of a product increases, it is required that the gate diameter of the molding mold be enlarged in order to flow the sufficient molten resin into the cavity of the mold. As the gate diameter becomes larger, the gate residual is stretched higher, and hence a tendency of occurrence of the high gate increases. Further, when speeding up a molding cycle, the molten resin in the gate portion is not well solidified, and therefore similarly the gate residual portion tends to increase in height.
When the gate residual portion 55 protrudes from the surface 54 of the shutter plate fitting portion 51a, there must be a trouble in fitting the shutter plate, which therefore needs a step of removing the protruded portion from the surface of the gate residual portion 55 by a post-process. As a result of an increased number of steps, the productivity declines.
Further, if the resin residual is attached to the gate portion, a flow of the molten resin is hindered, and consequently the cavity is not sufficiently supplied with the resin, with the result that a short shot tends to occur.
For preventing the above high gate, the gate diameter is, it can be considered, reduced down to, e.g. 0.5 mm. If reduced so, however, the short shot might easily occur.
The Japanese Patent Laid-Open Publication No. 5-293857 discloses a nozzle for the hot-runner of a thermoplastic resin mold constructed by, as shown in FIG. 2 therein, connecting a nozzle tip resin passage 2a of a hot-runner 2 of the thermoplastic resin mold to a straight pipe resin passage 5a of a spool 5 on a downstream side of the passage 2a by use of an eccentric gate 4, thereby preventing string stretch (resin residual) from occurring in the product.
The above publication shows no counter measure to the high gate and the short shot as shown in FIG. 8B.